The present invention relates to a method and apparatus for scanning a web material with a light beam, and more particularly to a scanning method and apparatus in which a web material being scanned is spatially divided into smaller sections in the direction of the width thereof.
Various methods and apparatus are well known for scanning a surface of a web material with a scanning beam in the direction of the width of the web material to detect surface defects of the web material. The detection of surface defects is effected on the basis of a light intensity fluctuation of the scanning beam either reflected by or transmitted through the web material. In the above-mentioned scanning method and apparatus, a rotary polyhedral mirror is generally used to move the scanning beam transversely over the web material from edge to edge. In such scanning method and apparatus, it is practically necessary to specify a transverse location of surface defect on the web material. For this reason, it is usual to divide spatially the surface of the web material into smaller sections in the direction of the width of the web material and to specify the location of a surface defect on the web material based on that section in which an excessive fluctuation of light intensity of the scanning beam occurs. In order to divide spatially the surface of the web material into smaller sections in the direction of the width thereof, it is heretofore usual to use clock pulses into which a series of standard pulses generated at a constant pulse repetition frequency is divided. With the aid of the clock pulses, the output in the form of light intensity distribution of the scanning beam is periodically spatially divided.
Alternatively, a single light beam is divided into two, namely a scanning beam and a reference beam, by means of a beam splitter such as a half mirror well known per se disposed between a rotary polyhedral mirror and a web material to be scanned. The reference beam is detected by means of a linear arrangement of a plurality of light detecting elements while the scanning beam scans the surface of the web material in the direction of the width thereof. The outputs from the light detecting elements divide an output in the form of the light intensity distribution of the scanning beam, thereby dividing spatially the surface of the web material in the direction of the width of the web material.
A problem of the conventional scanning methods and apparatus in which a rotary polyhedral mirror is used is that, as is well known to those skilled in the art, the scanning beam travels on the surface of a web material being scanned in the direction of the width of the web material at a varying speed of movement. It is, therefore, usual to use an f.theta. lens in order to make the scanning beam travel in the direction of the width of the web material at a constant speed of movement. The usage of such an f.theta. lens is disclosed in, for example, Japanese Laid-Open Utility Model No. 58-120,913; or "OPTICS", Vol 10, No. 5, October, 1981, especially "Optical Design of Laser Scanning Lens", by K. Minoura et al., page 348.
However, the provision of an f.theta. lens makes the scanning apparatus complex not only in construction but also in adjustment, and hence expensive. In addition, the scanning apparatus with an f.theta. lens cannot divide a web material being scanned into smaller divisions with different distances due to the use of clock pulses divided at a regular pulse repetition period.
On the other hand, the scanning apparatus in which the reference beam is used to divide the width of a web material being scanned into smaller sections requires the provision of light detectors as many in number as the number of the smaller sections to be divided. In addition, the use of the beam splitter for producing the scanning and reference beams from a single beam results in a reduced power of the scanning beam. For this reason, it is required to use a high power beam generator. This requirement, however, makes the scanning apparatus expensive and complex in construction.